Let me start this by saying I have very limited knowledge in biology and evolution.

However, when visiting museums and watching documentary movies, I always get impression dinosaurs (and other animals in "dinosaur era") are very huge compared to today's animals, especially land animals.

$\begingroup$You see them as unsuccessful? They roamed the planet for about 170 million years, which I find pretty impressive. If conditions change fast (like they did 66 Mio years ago) such animals cannot adapt fast enough.$\endgroup$
– Chris♦Oct 11 '18 at 13:55

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$\begingroup$Dinosaurs have been roaming the earth for 230 millions years. Birds are dinosaurs. Let's avoid fueling a weird non-monophyletic categorization to please a vague intuition.$\endgroup$
– Remi.bOct 11 '18 at 19:58

$\begingroup$You overlook one obvious factor. It's a lot easier to find big dinosaur bones (or bones of any sort) than little ones. I'd also guess that the bones of big creatures are more likely to be preserved in recognizable form...$\endgroup$
– jamesqfOct 12 '18 at 2:38

$\begingroup$@Chris I don't see them as unsuccessful. In fact, I'm very impressed by them. I am just curious was one trait of them, size, theirs disadvantage (especially when you compare them with todays land animals).$\endgroup$
– StupidOneOct 12 '18 at 15:23

2 Answers
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Dinosaurs were larger because their basic anatomy is better at being large than mammals. Their spine is better braced (no lumbar region), their breathing mechanism scales better, and their hip structure is stronger. This is called a preadaptation, or when an organism or group of organisms just starts with a head start on a particular adaptation due to unrelated reasons. It is the same reason mammals are the only terrestrial vertebrates to have echolocation: they are the only group with advanced amplification equipment in the ear to give it a head start.
Any era has its giants. Today is the time of the largest whales, largest trees, largest frogs, and largest horse to ever exist. Size is often a decent advantage: it makes you more resistant to resource fluctuation, it's just not a good for surviving global mass extinction inducing bolide impacts.

Evolution has no foresight. Evolution often produces and locks in counterproductive adaptations in the long run; while an adaptation may be an advantage in the short term, it may lock the organism into a disadvantaged position in the long run.
Sexual displays are a classic example: some quirk in peacock brains meant early female peacocks really liked more flamboyant males, that drove peacocks to the point modern males are incredibly vulnerable to predators and can barely fly due to carrying around huge superfluous tails. Both males and females would be better off preferring/having something less detrimental, but a feedback loop has locked in that detrimental feature. Males without it can't find mates, and females who do not prefer large tails male offspring also can't find mates. The term for this is fisherian or runaway selection. As an another example of counterproductive selection, many organisms when no longer controlled by predators or other factors will expand their numbers and consume resources to the point their populations collapse because there are too few resources to sustain them. Parasites often face this: a host with too many parasites dies, which is bad for the parasites but if the host can't fight off the parasites many are perfectly happy to kill their host and then starve.

Another aspect of evolution not having foresight is it cannot prepare you for extremely rare events, like mountains falling from the sky.

Some animals were big during the "dinosaur era" but not all and the largest animal to ever exist is extant today

Ancient dinosaurs have been around for 170 millions years. That's a very long time, much longer than anything you seem to be comparing to

Most lineages get extinct

NOT ALL DINOSAURS GOT EXTINCT! Some are still around

Semantic - On the concept of mistake

If you look to pretty much any definition of the term "mistake" (e.g. Merriam-Webster) you will find references to the concepts of choice and of intention. Indeed, in order to make a mistake one needs an intention, a goal.

Evolution is not some kind of mystical creature that has an intention for what ought to happen. Evolution has no intention. In this sense, it sounds very wrong to claim anything like "evolution made a mistake".

That makes your whole question (Has evolution made a mistake with prehistoric animals?) a little unclear.

Were pre-historic "huge?"

The concepts of historic vs pre-historic era are not concepts in empirical sciences but in history. The limit between the two is roughly placed at the invention of writing in humans. This boundary separate the past ~6000 years and the 13.8 billions years before it until the beginning of the universe (or the ~4 billions years of life on earth before it).

For the vast majority of these 4 billions years, life forms where definitely not all "huge". For most of this time, lifeforms were exclusively unicellular. First animals appeared ~550 millions years ago. And again, they were definitely not "huge" at the beginning. So a claim like "prehistoric animals are huge" would be very misleading.

Now of course, what you meant by "prehistoric" was actually "dinosaur era" but I still wanted to correct your choice of terms.

Were "dinosaur era" animals "huge"?

By "dinosaur era", someone probably refer to the period of ~170 millions years between the beginning of the triassic and the CP extinction event. Again, please note that in your comparison you are comparing a 66 millions years period to a 6 thousands years (0.006 millions years) period. To give you a little more perspective, there is about 170 millions years that separate a mid-triassic dinosaur (such as the Thrinaxodon) to a late Cretaceous dinosaur (such as the famous T-rex) but there is only ~66 millions years that separate a late Creataceous dinosaur to us, today!

Animal size varied a lot along these 66 millions years but yes, there were very big dinosaurs and also very large insects. This is mainly caused by a higher oxygen content in the atmosphere. Note however, that the largest animal that has ever lived is the blue whale who did not exist at that time but is extant today.

Extinction events

You probably meant to call "dinosaurs" a mistake because they do not exist today. First let me tell you that dinosaurs are not extinct, there are still a lot of dinosaurs alive today (see next section). Second, I would like to talk about extinction events.

Mass extinction events happen! In the phanerozoic only (that is in the past 500 millions years, that is a quarter of the time since life exist on earth), there are 7 commonly accepted mass extinction:

O-S

Late D

Cap

P-T

Tr-J

K-Pg

H

The last one, H refers to the holocene extinction and is the extinction event that is currently happening. Excluding the holocene extinction, the last mass extinction was the K-Pg (Cretaceous–Paleogene extinction event), which is the one who massively reduced the number of dinosaur lineages.

So really, don't think too much of modern life as being more "successful" that ancient dinosaurs because they are still extant. They just did not get extinct yet. If you want to call ancient dinosaurs a mistake, then you would have to call any lineage who got extinct a mistake and that represent the vast majority of all lineages that has ever existed. Also, these ancient dinosaurs have been around for 170 millions years! And also, ... as said above, dinosaurs are not extinct anyway!

Dinosaurs are not extinct

If by dinosaur, you mean Dinosauria, that is the monophyletic group that include all the things that you would typically call a dinosaur and all their descendent, then dinosaurs are still alive. Dinosaurs are still "flying around"! Birds are dinosaurs. They are not just distant cousins of dinosaurs but they are dinosaurs. It would indeed sound very wrong to state that dinosaurs are extinct just because we decided to name "bird" the dinosaur lineages that did not get extinct. For more information please have a look at the posts

$\begingroup$Whether or not you consider dinosaurs extinct depends on how you define "dinosaur". "Dinosaur" in the common sense refers to the extinct species.$\endgroup$
– AstrolambOct 11 '18 at 23:30

$\begingroup$That's right. I am using the only well defined definition that exists (the same used on wikipedia). I call a dinosaur any member of the clade Dinosauria$\endgroup$
– Remi.bOct 12 '18 at 1:04

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$\begingroup$Downvoted because of the "birds are dinosaurs" nonsense. By the same logic, birds, dinosaurs, and humans are just different kinds of fish :-)$\endgroup$
– jamesqfOct 12 '18 at 2:39

$\begingroup$Ha ha. Well, the point (maybe unclear; I tried to clarify that) is that stating dinosaurs are extinct is really silly because there are lineages that did not get extinct we just stopped calling them dinosaurs in the popular language. Nobody would say that all metazoa are extinct just because we got them more specific names.$\endgroup$
– Remi.bOct 12 '18 at 2:54

$\begingroup$Yeah, but what you've done is answer the question "what's wrong with this question [insert question]" in Semantics SE rather than try to answer the question that was asked in Biology, you could have probably condensed all the problems you have with it's wording into less than a 10th of this, defined what you thought he meant in less than that & then answered the question, which would have all been fine, you've only done the first bit ("your question is silly & this is why") & took a bleedin long time about it as well :)$\endgroup$
– PelinoreDec 20 '18 at 15:39